1. Field of the Invention
This invention relates to circuit interrupters including ground fault and/or arc fault trip mechanisms and, more particularly, to electronic trip units for circuit breakers and, more particularly, to such trip units, which respond to ground faults and sputtering arc faults. The invention also relates to trip signal generators for such trip units.
2. Background Information
Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 5,260,676; and 5,293,522.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which is heated and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature, which is attracted by the sizable magnetic forces generated by a short circuit or fault, also unlatches, or trips, the operating mechanism.
In many applications, the miniature circuit breaker also provides ground fault protection. Typically, an electronic circuit detects leakage of current to ground and generates a ground fault trip signal. This trip signal energizes a shunt trip solenoid, which unlatches the operating mechanism, typically through actuation of the thermal-magnetic trip device.
A common type of ground fault detection circuit is the dormant oscillator detector including first and second sensor coils. The line and neutral conductors of the protected circuit pass through the first sensor coil. The output of this coil is applied through a coupling capacitor to an operational amplifier followed by a window comparator having two reference values. A line-to-ground fault causes the magnitude of the amplified signal to exceed the magnitude of the reference values and, thus, generates a trip signal. At least the neutral conductor of the protected circuit passes through the second sensor coil. A neutral-to-ground fault couples the two detector coils which causes the amplifier to oscillate, thereby resulting in the generation of the trip signal. See, for example, U.S. Pat. Nos. 5,260,676; and 5,293,522.
Recently, there has been considerable interest in also providing protection against arc faults. Arc faults are intermittent high impedance faults which can be caused, for instance, by worn insulation between adjacent conductors, by exposed ends between broken conductors, by faulty connections, and in other situations where conducting elements are in close proximity. Because of their intermittent and high impedance nature, arc faults do not generate currents of either sufficient instantaneous magnitude or sufficient average RMS current to trip the conventional circuit interrupter. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads, which draw similar currents and would, therefore, cause nuisance trips. Consequently, separate electrical circuits have been developed for responding to arc faults. See, for example, U.S. Pat. Nos. 5,224,006; and 5,691,869.
Circuit interrupters, such as circuit breakers designed for arc fault applications including 5 mA ground fault protection (e.g., Underwriters Laboratory (UL) Standard UL 943) must be able to trip in either of the positive or negative half cycles in order to meet the requisite UL 943 trip times. This requires the use of a switching device, such as a triac, which has a gate drive requirement of at least about 3 mA. This, however, requires more power on a continuous basis than is practically available in a low cost product.
Integrated circuits (ICs) designed for ground fault and/or arc fault detection and protection produce a constant trip output when the trip condition is satisfied. This is acceptable when an SCR is used as the trip device because SCRs can be activated with a gate current of less than about 200 xcexcA. Moreover, miniature arc fault, ground fault, and/or arc fault/ground fault protection circuits in residential circuit breakers may need to operate with as little as about 3 mA of available DC power supply current. However, when a triac is employed (e.g., in order to meet the trip time requirements of the UL 943 standard), then higher levels of gate current are required in order to activate that device (e.g., about 5 mA, which is well above what current is available).
Expensive external circuits can be made to provide gate current only when the triac is in the OFF state. One arc fault/ground fault product requires significant and expensive external components to convert a continuous trip signal into a gated triac gate drive signal.
FIG. 1 is a block diagram, in schematic form, of a bi-directional trip circuit 2. The circuit 2 uses the gate trigger current from an arc fault/ground fault (AF/GF) detection circuit 4 to control a gate drive trigger circuit for triac 6. The triac 6, in turn, drives a trip solenoid coil 8. The circuit 2 is implemented as a daughter board for a main trip unit circuit board 10.
Accordingly, there is room for improvement in trip units and trip signal generators for arc fault and/or ground fault circuit interrupters.
These needs and others are met by the present invention. A trip signal generator for arc fault, ground fault and/or arc fault/ground fault circuit interrupters disables tripping until arc fault and/or ground fault detection circuits are fully operational, and then only generates a trip signal if a trip request is active and only if a semiconductor switching device is not active (i.e., is in the OFF state). This trip signal generator may generate suitably high gate drive signals of about 5 mA for a triac or 200 xcexcA for an SCR, while operating with an average available power supply current of less than about 2 mA.
As one aspect of the invention, a trip unit for an arc fault or ground fault circuit breaker comprises: a trip actuator having a trip coil; a semiconductor switching device driving the trip coil, with the semiconductor switching device including a gate and an output electrically interconnected with the trip coil; an arc fault or ground fault detection circuit including at least one output having an arc fault or ground fault detection signal; a power supply having an output; and a trip signal generator comprising: at least one first input electrically interconnected with the at least one output of the arc fault or ground fault detection circuit, a second input electrically interconnected with the output of the semiconductor switching device, a power supply monitor monitoring the output of the power supply and having an output, and a circuit having an output electrically interconnected with the gate of the semiconductor switching device, the circuit comprising: a first switch controlled by the second input and electrically interconnected with the output of the semiconductor switching device, the first switch having an output, a second switch controlled by the output of the first switch and powered from the output of the power supply monitor, the second switch having an output, a third switch controlled by the output of the second switch, the third switch powered from the output of the power supply, the third switch having the output electrically interconnected with the gate of the semiconductor switching device, and at least one fourth switch controlled by the at least one first input electrically interconnected with the at least one output of the arc fault or ground fault detection circuit, the at least one fourth switch enabling the third switch.
The output of the circuit may be electrically interconnected with the gate of a triac, and may be electrically interconnected with the gate of the triac by a resistor. The output of the circuit may source a current of at least about 5 mA to the gate of the triac through the resistor.
The trip signal generator may be an integrated circuit, such as a bipolar integrated circuit.
As another aspect of the invention, a trip signal generator for an arc fault or ground fault trip unit comprises at least one first input electrically interconnected with at least one output of an arc fault or ground fault detection circuit, a second input electrically interconnected with an output of a semiconductor switching device, which drives a trip coil of a trip actuator, with the semiconductor switching device including a gate and an output electrically interconnected with the trip coil, a power supply monitor monitoring an output of a power supply, the power supply monitor having an output, and a circuit having an output electrically interconnected with a gate of the semiconductor switching device, the circuit comprising: a first switch controlled by the second input and electrically interconnected with the output of the semiconductor switching device, with the first switch having an output, a second switch controlled by the output of the first switch and powered from the output of the power supply monitor, with the second switch having an output, a third switch controlled by the output of the second switch and powered from the output of the power supply, with the third switch having the output electrically interconnected with the gate of the semiconductor switching device, and at least one fourth switch controlled by the at least one first input electrically interconnected with the at least one output of the arc fault or ground fault detection circuit, with the at least one fourth switch enabling the third switch.
As another aspect of the invention, a trip signal generator for an arc fault or ground fault trip unit comprises at least one first input electrically interconnected with at least one output of an arc fault or ground fault detection circuit; a second input electrically interconnected with an anode of a semiconductor switching device, which drives a trip actuator; means for monitoring an output of a power supply and providing an output when the output of the power supply is operational; and means for driving a gate of the semiconductor switching device when the output of the power supply is operational, when the at least one first input is active, and when the second input is inactive.
The second input may be structured for electrical interconnection with the anode of a triac; and the means for driving the gate of the triac may generate a gate drive signal of about 5 mA. The trip signal generator may be structured for operation with an average current from the power supply of less than about 2 mA.